The present invention relates to a photodetecting device and, more particularly, to a photodetecting device having a polycrystalline superconductor film which has Josephson junctions at its grain boundaries. A photodetecting device utilizing Josephson junctions shown in FIG. 1 was described by Enomoto et al in a lecture article issued by the Institute of Electronics and Communication Engineers of Japan.
Referring to FIG. 1, reference numeral 1 denotes a cross-shaped oxide superconductor pattern of BaPb.sub.0.7 Bi.sub.0.3 0.sub.3 formed on a sapphire substrate 2 and having a thickness of 300 nm. The pattern 1 comprises a region 3, which is irradiated with light and located at the center of the pattern, a pair of input regions 4 and 4' extending from the region 3 to the left and right, and a pair of output regions 5 and 5' extending from the region 3 in the upper and lower directions. An input circuit (not shown) is connected to the input regions 4 and 4' to cause a DC bias current to flow thereto. An output circuit (not shown) including a differential amplifier for measuring changes in voltages generated between the output regions 5 and 5' is connected to the output regions 5 and 5'.
In the photodetecting device having the arrangement described above, when light irradiates the region 3 from an illuminating means (not shown) while a predetermined DC current flows from the input circuit to the pair of input regions 4 and 4', a change in voltage between the output regions 5 and 5' occurs. This voltage change is measured by the output circuit. In this manner, light can be detected. In general, the Josephson junctions require an electromagnetic shield and sufficient ground means to eliminate induction noise caused by background electromagnetic waves other than the target signal, since the Josephson junctions have high electromagnetic sensitivity.
However, in the photodetecting device shown in FIG. 1, the input and output circuits are independently arranged, so common ground is not present between the input and output circuits. Thus, it is difficult to decrease the induction noise level to a target level.